Aromatic substances are roughly classified into two categories: that is, chemically synthesized (so-called “synthetic aromatics”); and non-chemically synthesized (so-called “natural aromatics”), depending on the starting material thereof or the method for producing the same. Recently, consumers tend to prefer “natural products” over “synthetic products.” However, natural substances contain only very small amounts of, for example, optically active substances of γ-decalactone (R-γ-decalactone or S-γ-decalactone) and δ-decalactone, which are important ingredients of natural food flavor. Thus, processes such as extraction of such optically active substances with high optical purity or isolation thereof via other means are disadvantageous from technical and economical viewpoints. As a result, large quantities of synthetic products are generally supplied at low prices at present. In contrast, the scale of manufacturing natural products is small, and such natural products are often expensive.
Development of a method for supplying large quantities of the aforementioned natural aromatics at prices as low as those of currently used synthetic aromatics has been awaited. Among the methods for supplying large quantities of natural aromatics that have been proposed a microorganism-based fermentation technique has drawn attention. In this technique, natural R-γ-decalactone is produced from natural materials or degradation products thereof via biological or physical techniques without any chemical techniques.
For example, JP Patent Publication (Kokai) No. 59-82090 A (1984) discloses a microorganism-based method for producing γ-decalactone from castor oil or a hydrolysate thereof. In this method, microorganisms such as Aspergillus oryzae, Candida rugosa, Geotrichum klebannii, and Yarrowia lipolytica are used to produce γ-hydroxydecanoic acid, and the resultant is acidified with the addition of hydrochloric acid or the like, followed by heating for lactonization, and thus γ-decalactone is produced. JP Patent Publication (Kokai) No.63-56295 A (1988) and the report by K. A. Maume et al. (Biocatalysis, vol. 5, 79–97, 1991) disclose a method for producing γ-decalactone wherein γ-hydroxydecanoic acid is produced from ricinoleic acid sources using Sporobolomyces odorus or Rhodotorula glutinis, and the resultant is also lactonized. JP Patent Publication No. 2-174685 A (1990) discloses a method for producing γ-decalactone from castor oil or ricinoleic acid via production of γ-hydroxydecanoic acid with the use of microorganisms such as Aspergillus niger. JP Patent Publication No. 3-117494 A (1991) discloses the same technique with the use of microorganisms such as Saccharomyces cerevisiae. Prior to the disclosure of these techniques, S. Okui et al. reported the presence of γ-hydroxydecanoic acid and γ-decalactone as intermediates during the process for oxidizing and degrading ricinoleic acid with the use of several cell strains of the genus Candida (J. Biochem., vol. 54, No. 6, 536–540, 1963). Also, EP 997533 discloses a method for producing γ-decalactone from castor oil at 12 g per liter using Yarrowia lipolytica. This method for production, however, is extremely disadvantageous from the viewpoint of production efficiency due to the necessity of the use of an emulsifier or pH adjuster during the culture and a small amount of a starting material, i.e., castor oil, to be added to the culture system, which is as low as 0.0247 kg/L. Further, the microorganisms disclosed in such publications and the like, which are of species different from those of the microorganisms used in the present invention, are not always suitable for practical use because of the difficulty of separation of cell strains from culture products and insufficient amounts of production from an economical viewpoint.
Alternatively, a method for producing γ-decalactone from a sugar substrate with the aid of Sporobolomyces odorus (S. Taqhara et al., Agric. Biol. Chem., vol. 36, No. 13, 2585–2587, 1972; N. Jourdain et al., “Top. Flavour Res., Proc. Int. Conf,” H. Eichhorn, 427–441, 1985) or Fusarium poae (J. Sarris et al., Agric. Biol. chem., vol. 49, No. 11, 3227–3230, 1985), which is an example of methods in which a component other than castor oil or a hydrolysate thereof is employed as a carbon source, has been reported. However, these techniques are not suitable for industrial-scale production since only a very small amount of γ-decalactone is produced.
Accordingly, development of a method for effectively producing γ-hydroxydecanoic acid and γ-decalactone that does not require the use of an emulsifier or pH adjuster and that allows the addition of a highly concentrated starting material, i.e., castor oil and/or a hydrolysate thereof, has been awaited.
It is also reported that the abundance of R-γ-decalactone enantiomer in naturally occurring γ-decalactone is excessive (A. Bernreuther et al., J. Chromatography, 481, 363, 1989). A method for producing a pure optically active form of such R-γ-decalactone via chemical synthesis is disclosed in JP Patent Publication No. 4-108782 A (1992).
R-γ-decalactone can also be produced by selectively separating R-γ-decalactone from racemic mixtures as extracted from natural substances by a technique known to a person skilled in the art. Because of the very small amount of R-γ-decalactone in natural substances and a physical difficulty in separating R-γ-decalactone from other volatile compounds, however, extraction thereof from natural substances is not cost-effective. In order to deal with increasing demands for natural compounds as mentioned above, development of a method for effectively producing natural R-γ-decalactone using techniques different from the chemical synthesis thereof or the separation thereof from racemic mixtures, has been awaited.
A method for producing δ-decalactone using microorganisms, which makes use of the reducing ability of fungi, particularly that of yeast, has been proposed. For example. JP Patent Publication No. 3-155792 A (1991) discloses a method for producing 5-decanolide from naturally occurring 2-decen-1,5-olide with the utilization of the reducing ability of Saccharomyces cerevisiae. JP Patent Publication No. 6-225781 A (1994) reports a method for producing δ-decanolide, δ-dodecanolide, or a mixture thereof from a substrate material containing a corresponding unsaturated lactone, i.e., δ-decen-2-olide, δ-dodecen-2-olide, or a mixture thereof, via biohydrogenation with the use of yeast such as Saccharomyces delbrueckii. However, a technique of chemical conversion utilizing the reducing ability of yeast is still problematic in terms of, for example, the difficulty in acting on high concentrations of the substrate and the necessity of a long period of time to obtain the substance of interest.